Porous films are well-suited to chemical sensing, as the optical response of the films shifts in a predetermined manner. However, ambient conditions can cause drift over time. Humidity, in particular, affects the baseline optical signal from a porous film, such as porous silicon, which can interfere with the ability to detect an analyte of interest via optical methods. The base line signal of a porous silicon film can also drift over time as oxidation caused by exposure to air reduces the natural hydrophobicity of an etched porous silicon layer over time, increasing the response of a porous silicon sensor to water vapor. The oxidation reaction thus leads to significant zero point drift. Another problem with optical interrogation of porous films concerns off-angle measurements. This is particularly problematic in remote sensing, where the angle of sensing is difficult to guarantee and replicate. A porous film that is optically interrogated at different angles can produce different signals. The variance in signals caused by angular variance in interrogation also interferes with the ability to detect or identify analytes of interest.
Optical reflectance is the most commonly used method used to interrogate porous sensors, and provides a convenient transduction mechanism that can be probed at a distance, either in free space or at the distal end of an optical fiber for remote detection of chemicals. In a free-space optical configuration, the intensity of one or more wavelengths of light reflected from the porous Si-based photonic structure is monitored using a laser or incoherent light source. Reflected intensity depends on the refractive index of the porous matrix, which is related to its chemical composition. Any adsorbate or chemical reaction that alters the chemical composition can, in principle, be detected. However, problems arise with signal drift as discussed above. In addition, the need to detect a reflectance spectrum requires optical sensing equipment that limits application.
A double layer sensor has been used to compensate for humidity drift. Ruminski, Moore & Sailor, “Humidity-Compensating Sensor for Volatile Organic Compounds Using Stacked Porous Silicon Photonic Crystals,” discloses double stack porous silicon sensors having a hydrophobic top stack and a hydrophilic bottom stack. The optical spectrum of the double-stack structure provides an effective structure to discriminate VOCs (volatile organic contaminants) from water vapor. Shifts in the peak frequencies from both photonic crystal stacks in the sensor are measured simultaneously. The hydrophilic and hydrophobic stacks respond differently to water and to VOC. The different response and the simultaneous measurement permits the effect of changing humidity to be nulled by calculating the weighted difference between the two peak frequencies. The method requires determination of a constant nulling factor for the double stack sensor.
Porous sensors can also be made chemically specific. Porous Si photonic crystals, for example, can be modified to incorporate a chemical reaction that is specific for an individual analyte or for a class of similar analytes. Oxidized porous Si is normally stable in air, but traces of HF(g) react with and remove the surface oxide, generating a blue shift in the reflectance spectrum. This reaction can be coupled to other reactions that produce HF, for example, the catalytic hydrolysis of fluorophosphonate nerve agents by copper-containing catalysts. Incorporation of a copper-based hydrolysis catalyst in a porous SiO2 matrix generates a sensor that is specific for the P—F bonds of the nerve agent sarin and related fluorophosphonates. See, e.g., H. Sohn, S. Létant, M. J. Sailor, W. C. Trogler, J. Am. Chem. Soc. 122, 5399 (2000). Another surface chemistry that is selective for certain classes of chemicals is silicon hydride. The electrochemical preparation of porous Si generates a surface that is covered with Si—H, SiH2, and SiH3 species. Though kinetically stable in air, this surface is rapidly degraded in corrosive environments (e.g. O2, O3, Cl2, NOx). The reactions convert Si—Si and Si—H bonds to Si—O, which results in a decrease in refractive index of the porous Si matrix and a characteristic blue shift in the reflectance spectrum.
Detection of HF in environmental samples is commonly accomplished not with porous silicon but instead using a collection agent that is exposed for a prescribed period of time and subsequently subjected to a laboratory-based analysis. For example, filter paper impregnated with K2HPO4 can be used as a collection agent; after exposure the sample is eluted with 0.1 M sodium citrate and the fluoride concentration is determined potentiometrically. This method can detect HF gas in the concentration range 0.68-5.45 ppm, over a period of 48 h. Polymer-based collection agents have also been employed; an alkaline impregnated polypropylene film detects HF gas in the concentration range 0.1-387 ppm in 4 h when subjected to electrochemical analysis using a fluoride selective-ion electrode. These dosimeters are designed to respond to HF in the gas phase only, and cannot detect aqueous hydrofluoric acid. Also, the methods do not monitor HF in real-time, but must be analyzed after exposure.
A common problem with porous sensors in general and any of the specific porous sensors discussed above is the aformentioned humidity and off angle measurement problems. Either or both of these issues can result in significant errors associated with detection of chemicals of interest in the environment using photonic materials, specifically, zero point drift of the measured spectrum and dependence of the spectrum on the observer-sample angle.